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The 90/10 ruleThis rule, policed and enforced by the Department of Education, states that for-profit schools can derive no more than 90% of their yearly revenue from Title IV funds. These funds represent the bulk of government aid granted for the purpose of attending college (public, private, and for-profit), and are more commonly referred to by their specific names: Stafford loans, Perkins loans, Pell grants, etc.

In other words, to make sure an organization isn't just swindling the government, students collectively must cover 10% or more of the school's revenue without assistance from the government.

The rule itself shows how much risk investors take on when investing in for-profits. When so much revenue is derived from one customer (here, the government), schools are at the mercy of that customer.

The lower a school's numbers in the table below, the better position it is in. Here's how the companies currently stack up, according to their Securities and Exchange Commission filings.

School

90/10 standing

Corinthian Colleges(Nasdaq: COCO)

89.8%

Apollo Group(Nasdaq: APOL)

88.0%

Bridgepoint Education(NYSE: BPI)

85.0%

Strayer Education(Nasdaq: STRA)

78.0%*

Education Management(Nasdaq: EDMC)

77.0%

ITT Tech(NYSE: ESI)

59.0%

American Public(Nasdaq: APEI)

19.0%

Source: SEC Filings. *For 2009.

The danger zoneThe numbers above represent the total weighted amount of revenue that comes from Title IV. But instead of using this measure, the government goes on a campus-by-campus basis. This could spell doom for some schools.

Take Corinthian, for example. While it was able to collectively pass the 90% rule by a razor-thin 0.2%, 42 of its 49 campuses exceed the 90% threshold, meaning that these campuses will lose federal funding if they don't get their act together within two years. Even more suspect is the fact that Corinthian recently raised tuition, forcing students to pay more through private loans, in order to duck beneath the 90% threshold.

Apollo and Bridgepoint have also publicly stated that, because of recent increases in Title IV grant and loan limits, they will be flirting dangerously close to the 90% threshold during this fiscal year.

The rest of the packWhile Strayer, Education Management, and ITT seem to be safe for now, they are subject to the same pressures that all industry players face.

The company that stands out, though, is American Public Education. Functioning primarily as an option for officers in the military to obtain their degrees, it has been able to avoid getting into potential trouble with the 90/10 rule.

That may not, however, be the case for very long. Some members of Congress have stated a desire for money from the Department of Defense's tuition assistance program to count toward the government assistance total. As of now, such funding does not count against American Public. Because its students are predominantly from the armed forces, such tuition assistance accounts for a large portion of overall revenue. There's no specific timeline for when such changes may even be discussed.

Foolish takeawayAs you can probably see by now, when free-market economics become entangled in government regulation, it can be difficult to project into the future with any level of certainty. It remains to be seen whether the Department of Education will follow through on closing campuses that exceed the 90% rule. It would surely put hundreds of school staff out of a job, and that's the type of action that government agencies are loath to take.

We're more than halfway to finding our diamond in the rough. Though today didn't yield any clear leaders, American Public is attractive with the funding rules as they are today.

Check in tomorrow. I'll be discussing what I see to be the most important metric to evaluate: student-loan default rates.

In the meantime, if you have specific thoughts on the for-profit education industry, leave a comment below and help us form a fuller picture of the challenges facing the industry.

Fool contributor Brian Stoffel does not own shares in any of the companies mentioned in this article.Motley Fool Optionshas recommended a call spread position on Bridgepoint Education. The Fool owns shares of American Public Education, Bridgepoint Education, and Strayer Education.